GCSE · Biology · Edexcel · Spec 1BI0

Nephron structure and function

A urine test finds no glucose and no protein. Same result, two completely different reasons. Follow the filtrate and you'll see both.

Follow one batch of filtrate

Tap each stage and ask two questions: what leaves here, and what goes back into the blood? Stage 6 is the blood's side of the story.

Biology · Detective work

Where did the glucose go?

A urine sample is tested and it contains no glucose.

Which explanation is closest to what you think?

Glucose never gets filtered out of the blood, so it never gets near the urine.Glucose is missing for the same reason protein is: it is too big to be filtered.Glucose is filtered, but it gets lost somewhere in the kidney before the urine forms.Glucose is filtered, then all of it is reabsorbed in the proximal convoluted tubule, so none reaches the urine.

How sure are you?

I'm sureI think soI'm guessing

Glucose never gets filtered out of the blood, so it never gets near the urine.

This is a common misconception

Easy trap, because protein really is never filtered. But glucose is one of the things filtered out of the blood plasma at the glomerulus and Bowman's capsule. It does get into the tubule, which is why it is in the filtrate.

Glucose is missing for the same reason protein is: it is too big to be filtered.

This is a common misconception

Only blood cells and large proteins are too big to be filtered. Glucose is filtered along with ions, water and urea. Glucose and protein are both missing from urine, but for two different reasons.

Glucose is filtered, but it gets lost somewhere in the kidney before the urine forms.

This is a common misconception

It isn't lost. In the proximal convoluted tubule all of the glucose is reabsorbed, which means taken back into the blood. It leaves the kidney in the blood, not in the urine.

Glucose is filtered, then all of it is reabsorbed in the proximal convoluted tubule, so none reaches the urine.

This matches the evidence

That's the one. Filtered at the glomerulus and Bowman's capsule, all reabsorbed in the proximal convoluted tubule, so it is in the filtrate but not in the urine. Protein, by contrast, is too big to be filtered, so it never enters the tubule at all.

Glucose: filtered, then all reabsorbed. Protein: never filtered. Same missing result, two different reasons.

Predict, then check

Two people, two very different days. Commit to a prediction before you look.

Alex has just drunk a huge amount of water, so the body has excess water. Priya has had nothing to drink all day, so her body's water balance is low. Which pair of urine samples fits?

Biology · Scale

How big is the filtering job?

How many nephrons do you think are in ONE kidney?

Your estimate

2500000 nephrons

How many nephrons do you think are in ONE kidney? Estimate between 0 nephrons and 5000000 nephrons.

0 nephrons5000000 nephrons

Biology · In your own words

Write the explanation

Explain why glucose is found in the filtrate but not in the urine. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Glucose is filtered out of the blood plasma into the tubule at the glomerulus and Bowman's capsule. (1 mark)
All of the glucose is reabsorbed back into the blood. (1 mark)
This happens in the proximal convoluted tubule, so none reaches the collecting duct to be part of the urine. (1 mark)

Model answer

Glucose is filtered out of the blood plasma into the tubule at the glomerulus and Bowman's capsule, so it is in the filtrate. In the proximal convoluted tubule all of the glucose is reabsorbed back into the blood. None is left to reach the collecting duct, so it is not in the urine.

Say it in order: filtered, then all reabsorbed in the proximal convoluted tubule, so none reaches the urine.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Filter first. Take back second. Whatever is left is urine.

What you need to know

Each kidney has about one million tiny tubules called nephrons, which filter the blood and carry out selective reabsorption.
A nephron runs in this order: glomerulus and Bowman's capsule, proximal convoluted tubule, loop of Henle, collecting duct.
Blood arrives through the renal artery, and the filtered blood leaves through the renal vein.
At the glomerulus and Bowman's capsule, glucose, ions, water and urea are filtered out of the blood plasma into the tubule.
Blood cells and large proteins are too big to be filtered, so protein is found in neither the filtrate nor the urine.
Have a goYour mate Sam insists: "A blood cell squeezed into the tubule, so it'll turn up in the urine." Where does Sam's story go wrong?
Blood cells never get into the tubule. They're too big to be filtered.
Filtering lets glucose, ions, water and urea through, but blood cells and large proteins stay in the blood.
In the proximal convoluted tubule, all of the glucose and some of the ions are reabsorbed back into the blood.
Have a goSay 50 units of glucose are filtered into one tubule. How many of those units end up in the urine?
None. All 50 units go back into the blood.
The proximal convoluted tubule reabsorbs all of the glucose, so none is left for the urine. 'Some is lost' is the tempting wrong answer.
In the loop of Henle, water is reabsorbed into the blood.
How much water and how many mineral ions are reabsorbed varies, and that is selective reabsorption. All of the glucose is always reabsorbed.
Have a goPick one: which can change from one situation to the next, the amount of water reabsorbed or the amount of glucose reabsorbed?
The amount of water.
The amount of water and mineral ions reabsorbed varies, but all of the glucose is always reabsorbed.
The collecting duct collects the urea, excess water and excess ions as urine, which drains through the ureter to the bladder for storage.
Low water balance means more water is reabsorbed, giving a small volume of concentrated urine. Excess water means a large volume of dilute urine.

The big picture

A nephron is a tube that filters the blood and then takes back what the body keeps. Glucose, ions, water and urea are filtered out of the plasma; blood cells and large proteins are too big to be filtered. All of the glucose and some ions go back at the proximal convoluted tubule, water goes back at the loop of Henle, and what the collecting duct gathers is urine.

Key points

1A nephron filters the blood at the glomerulus and Bowman's capsule, then reabsorbs useful substances on the way along the tube.

2Glucose, ions, water and urea are filtered out of the blood plasma. Blood cells and large proteins are too big to be filtered.

3All of the glucose and some of the ions are reabsorbed in the proximal convoluted tubule. Water is reabsorbed in the loop of Henle.

4The amount of water and mineral ions reabsorbed varies (selective reabsorption). The amount of glucose reabsorbed does not vary: it is all of it.

5The collecting duct collects urea, excess water and excess ions as urine, which passes down the ureter to the bladder.

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Worked example

Problem

Two water molecules are filtered at the same moment. Molecule A is reabsorbed and ends up leaving the kidney in the renal vein. Molecule B ends up in the urine. Trace each journey and say what decides the difference.

⚠ Watch out

Mixing up filtering and reabsorbing. Filtering takes things out of the blood and into the tubule. Reabsorbing is the reverse: taking things from the filtrate back into the blood. Glucose gets both treatments, in that order, which is why it is filtered but still not in the urine.

🧠

Memory hook

Order of the tube: Glomerulus, Proximal, Loop, Collecting. "Go Pick Lovely Carrots." And the rule that goes with it: filter first, take back second, what's left is urine.

✓

Check yourself

Without looking back: name the four parts of a nephron in order, then say which one takes back all of the glucose and which one takes back water.

Flashcards

(11)

What is a nephron?

One of about a million tiny tubules in each kidney. Nephrons filter the blood and carry out selective reabsorption.

Name the parts of a nephron in order.

Glomerulus and Bowman's capsule, then the proximal convoluted tubule, then the loop of Henle, then the collecting duct.

Which substances are filtered out of the blood plasma at the glomerulus and Bowman's capsule?

Glucose, ions, water and urea.

Why is there no protein in the filtrate or the urine?

Blood cells and large proteins are too big to be filtered, so protein stays in the blood.

Which two things come back into the blood in the proximal convoluted tubule?

All of the glucose, and some of the ions.

Where in the nephron is water reabsorbed into the blood?

In the loop of Henle.

What is selective reabsorption?

The amount of water and mineral ions reabsorbed varies. The glucose is different: all of it is always reabsorbed.

What does the collecting duct collect?

The urea, the excess water and the excess ions. Together they form urine.

Where does urine go after the collecting ducts?

Down the ureter to the bladder, where it is stored.

Which blood vessel brings blood into the kidney, and which takes it away?

The renal artery brings it in. The renal vein takes the filtered blood away, back into circulation.

How does the body's water balance change the urine?

Low water balance: more water reabsorbed, so a small volume of concentrated urine. Excess water: more water excreted, so a large volume of dilute urine.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Nephron structure and function

Who makes it into the urine?

Where did the glucose go?

How big is the filtering job?

Write the explanation

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